The present invention relates to a hydraulic system for an automatic transmission, and more particularly, to a hydraulic system for an automatic transmission for a vehicle having idle-stop control for stopping engine idle at a standstill of the running vehicle.
In recent years, idle-stop vehicles are already operational wherein when the running vehicle comes into a standstill, and predetermined stop conditions are formed, an engine is automatically stopped to achieve fuel savings, a reduction in exhaust emission or noises and the like. With such vehicle, when the engine is stopped, a main pump driven by the engine is stopped, so that oil supplied to a forward engagement element of an automatic transmission is drawn from a hydraulic passage, lowering the hydraulic pressure. As a result, when the engine is restarted, the forward engagement element to be engaged at forward running releases from its engagement state. Thus, if the forward engagement element is not engaged quickly at engine restart, an accelerator pedal is depressed in the neutral state as it were, which may produce an engagement shock by engagement of the forward engagement element with the engine at full throttle.
The technique for solving the above inconvenience is proposed in JP-A 2000-46166. This technique uses two pumps wherein the main pump is operated to supply hydraulic fluid, and the assist pump driven by an electric motor is operated solely at stop of the main pump such as at engine stop to compensate for flow shortage, allowing secured supply of hydraulic fluid to the automatic transmission while keeping power consumption of a battery at the minimum.
However, in the above earlier art, the assist pump driven by the electric motor is always driven at engine stop, leading to significant power consumption. At traffic congestion or the like, particularly, there arises a problem that a great load is applied not only to the battery, but also to the motor itself for driving the assist pump. Moreover, since the assist pump is driven by the electric motor, the assist pump and the electric motor should be mounted on the automatic transmission, raising a problem of deteriorating the vehicle mountability.
It is, therefore, an object of the present invention to provide a hydraulic system for an automatic transmission for a vehicle having idle-stop control, which allows smooth running of the vehicle during execution of idle-stop control.
The present invention provides generally a hydraulic system for a vehicle with: an engine having idle-stop control, the idle-stop control controlling idle-start and idle-stop of the engine in accordance with preset idle-stop conditions; and an automatic transmission which carries out speed change using a hydraulic pressure supplied from a hydraulic-pressure source, the automatic transmission comprising a forward engagement element, wherein the hydraulic system comprises: a control valve unit which controls the speed change of the automatic transmission, the control valve unit including a manual valve; a bypass passage which allows communication between a first hydraulic passage downstream of the manual valve and the forward engagement element; a first switching valve arranged on the bypass passage, the first switching valve being switched between a communicating state and a non-communicating state; and a first switching device which switches the first switching valve between the communicating state and the non-communicating state, the first switching device switching the first switching valve to the communicating state when the hydraulic pressure is lower than a predetermined pressure.